Hi, I recently picked up a OS Max .25 at a garage sale. It was seized and missing a carb, but I managed to resolve both issues (it's got a new OS 2A carb now).

I believe it's the FP model i.e. no remote needle valve, though it may very well be even older. After a good cleaning and the new carb, all seems well - bearings are smooth, piston feels tight with good compression.

Just started it first flick on Omega 5% with extra castor added and all seems well BUT on prolonged WOT, it does seem rather hot. I know it's getting less cooling on the bench and it hasn't gotten hot enough to cut out, but my spittle does sizzle on the heat sink. I remember that this is a no no from my limited experience running RC cars.

I'm running it rich enough to see white smoke at WOT, but its definitely in 2 stroke mode. Power seems good and the engine note is generally steady both at WOT and idle. I'm turning a 9 x 5 Master Airscrew prop at the moment (Based on the 9 x 4 run in prop recommended for the .25 LA) Heat aside, the engine seems to be runnning really good - plenty of power and reasonably smooth transitions.

Is the engine running too hot? Should I be worried or at least start thinking about changing the sleeve or something else? Perhaps prop down to lower the load?

I was wondering along similar lines about my .25FP. Even with full castor it seems to run pretty hot, but it's hard to judge exactly how hot. I do admit that I don't the actual oil percentage of my fuel though.

Btw, efish: if you have an FP, AFAIK it will say so on the side, otherwise it's an older version.

Would anyone know if it's possible / worth the bother to upgrade the sleeve and/or piston? I can get OS parts for very reasonable prices, so if it can be done, I might give it a go - fuel grade castor oil is really pricey here, so a new piston sleeve will likely pay for itself very quickly!

How hot is hot ?
If you have a chance to measure it, than around 230 to 250 F is a normal range for head temperature. Did you "measure" static, meaning "not flying" ?
After a flight, if your head temperatue is "Ok to touch with your hand" then you're OK.

I had a 25FP years ago on a trainer I if I remember right it was a bushed ABC engine. I ran 10% Redmax fuel in it for years without a problem. Can't comment on the heat at WOF as mine never saw that on the ground. when the plane landed you could touch the cylinder head for briefly without cooking your finger, but keep in mind that was usually after a period of low throttle or idling for landings. I say if it keeps smoking and runs fine otherwise (mixture doens't change due to heat) then you're good to go.

many times too much heat is cause by a too lean setting. 95% of flyers set their needle valves for max rpm on the ground. WRONG!
The needle valve should be set to just break into the 2 cycle mode and should not sag in rpm when the model is held straight up and....
that's when you need the max power.
5% lean setting - 30% loss in power
5% rich setting - probably never notice it

also many schuerle manufacturers made their sleeves shiny and slick. During break in if you get just one lean run it can cause huge heat problems. The cylinder wall must have some tiny horizontal scratches in it so as to hold oil on that cylinder wall.
too much heat?
first check for lose head gasket..... next leaking backplate gasket and most certainly never ignore that the engine is running too lean from many causes... pin hole in tank or fuel line... line pinched... tank too low or feed line too long. Carbuerator mechanism loose or leaking air around the needle valve or carbuerator barrel are some of the many reasons.

Hi Bill, thanks for the tips. Did check for an air leak previously by spritzing water around the areas you mentioned. All seems well.

The point about running rish is taken - always set my needle valve just past the point where the engine is transitioning between 2 and 4 stroke. I'm a casual sport flyer, so I don't need the edge in power. Would much rather have my engine/piston/cylinder/plugs last longer.